png BMP, JPEG, PNG and TIFF graphics devices

Description

Graphics devices for BMP, JPEG, PNG and TIFF format bitmap files.

Usage

bmp(filename = "Rplot%03d.bmp",
    width = 480, height = 480, units = "px", pointsize = 12,
    bg = "white", res = NA, ...,
    type = c("cairo", "Xlib", "quartz"), antialias)

jpeg(filename = "Rplot%03d.jpeg",
     width = 480, height = 480, units = "px", pointsize = 12,
     quality = 75,
     bg = "white", res = NA, ...,
     type = c("cairo", "Xlib", "quartz"), antialias)

png(filename = "Rplot%03d.png",
    width = 480, height = 480, units = "px", pointsize = 12,
     bg = "white",  res = NA, ...,
    type = c("cairo", "cairo-png", "Xlib", "quartz"), antialias)

tiff(filename = "Rplot%03d.tiff",
     width = 480, height = 480, units = "px", pointsize = 12,
     compression = c("none", "rle", "lzw", "jpeg", "zip", "lzw+p", "zip+p"),
     bg = "white", res = NA,  ...,
     type = c("cairo", "Xlib", "quartz"), antialias)

Arguments

filename

the output file path. The page number is substituted if a C integer format is included in the character string, as in the default. (The result must be less than PATH_MAX characters long, and may be truncated if not. See postscript for further details.) Tilde expansion is performed where supported by the platform. An input with a marked encoding is converted to the native encoding on an error is given.
width

the width of the device.
height

the height of the device.
units

The units in which height and width are given. Can be px (pixels, the default), in (inches), cm or mm.
pointsize

the default pointsize of plotted text, interpreted as big points (1/72 inch) at res ppi.
bg

the initial background colour: can be overridden by setting par("bg").
quality

the ‘quality’ of the JPEG image, as a percentage. Smaller values will give more compression but also more degradation of the image.
compression

the type of compression to be used. Ignored for type = "quartz".
res

The nominal resolution in ppi which will be recorded in the bitmap file, if a positive integer. Also used for units other than the default, and to convert points to pixels.
...

for type = "Xlib" only, additional arguments to the underlying X11 device such as fonts or family.

For types "cairo" and "quartz", the family argument can be supplied. See the ‘Cairo fonts’ section in the help for X11.

For type "cairo", the symbolfamily argument can be supplied. See X11.options.

type

character string, one of "Xlib" or "quartz" (some macOS builds) or "cairo". The latter will only be available if the system was compiled with support for cairo – otherwise "Xlib" will be used. The default is set by getOption("bitmapType") – the ‘out of the box’ default is "quartz" or "cairo" where available, otherwise "Xlib".
antialias

for type = "cairo", giving the type of anti-aliasing (if any) to be used for fonts and lines (but not fills). See X11. The default is set by X11.options. Also for type = "quartz", where antialiasing is used unless antialias = "none".

Details

Plots in PNG and JPEG format can easily be converted to many other bitmap formats, and both can be displayed in modern web browsers. The PNG format is lossless and is best for line diagrams and blocks of colour. The JPEG format is lossy, but may be useful for image plots, for example. BMP is a standard format on Windows. TIFF is a meta-format: the default format written by tiff is lossless and stores RGB (and alpha where appropriate) values uncompressed—such files are widely accepted, which is their main virtue over PNG.

png supports transparent backgrounds: use bg = "transparent". (Not all PNG viewers render files with transparency correctly.) When transparency is in use in the type = "Xlib" variant a very light grey is used as the background and so appears as transparent if used in the plot. This allows opaque white to be used, as in the example. The type = "cairo", type = "cairo-png" and type = "quartz" variants allow semi-transparent colours, including on a transparent or semi-transparent background.

tiff with types "cairo" and "quartz" supports semi-transparent colours, including on a transparent or semi-transparent background. Compression type "zip" is ‘deflate (Adobe-style)’. Compression types "lzw+p" and "zip+p" use horizontal differencing (‘differencing predictor’, section 14 of the TIFF specification) in combination with the compression method, which is effective for continuous-tone images, especially colour ones.

R can be compiled without support for some or all of the types for each of these devices: this will be reported if you attempt to use them on a system where they are not supported. For type = "Xlib" they may not be usable unless the X11 display is available to the owner of the R process. type = "cairo" requires cairo 1.2 or later. type = "quartz" uses the quartz device and so is only available where that is (on some macOS builds: see capabilities("aqua")).

By default no resolution is recorded in the file, except for BMP. Viewers will often assume a nominal resolution of 72 ppi when none is recorded. As resolutions in PNG files are recorded in pixels/metre, the reported ppi value will be changed slightly.

For graphics parameters that make use of dimensions in inches (including font sizes in points) the resolution used is res (or 72 ppi if unset).

png will normally use a palette if there are less than 256 colours on the page, and record a 24-bit RGB file otherwise (or a 32-bit ARGB file if type = "cairo" and non-opaque colours are used). However, type = "cairo-png" uses cairographics' PNG backend which will never use a palette and normally creates a larger 32-bit ARGB file—this may work better for specialist uses with semi-transparent colours.

Quartz-produced PNG and TIFF plots with a transparent background are recorded with a dark grey matte which will show up in some viewers, including Preview on macOS.

Unknown resolutions in BMP files are recorded as 72 ppi.

Value

A plot device is opened: nothing is returned to the R interpreter.

Warnings

Note that by default the width and height values are in pixels not inches. A warning will be issued if both are less than 20.

If you plot more than one page on one of these devices and do not include something like %d for the sequence number in file, the file will contain the last page plotted.

Differences between OSes

These functions are interfaces to three or more different underlying devices.

  • On Windows, devices based on plotting to a hidden screen using Windows' GDI calls.

  • On platforms with support for X11, plotting to a hidden X11 display.

  • On macOS when working at the console and when R is compiled with suitable support, using Apple's Quartz plotting system.

  • Where support has been compiled in for cairographics, plotting on cairo surfaces. This may use the native platform support for fonts, or it may use fontconfig to support a wide range of font formats.

Inevitably there will be differences between the options supported and output produced. Perhaps the most important are support for antialiased fonts and semi-transparent colours: the best results are likely to be obtained with the cairo- or Quartz-based devices where available.

The default extensions are ‘.jpg’ and ‘.tif’ on Windows, and ‘.jpeg’ and ‘.tiff’ elsewhere.

Conventions

This section describes the implementation of the conventions for graphics devices set out in the ‘R Internals’ manual.

  • The default device size is in pixels.

  • Font sizes are in big points interpreted at res ppi.

  • The default font family is Helvetica.

  • Line widths in 1/96 inch (interpreted at res ppi), minimum one pixel for type = "Xlib", 0.01 for type = "cairo".

  • For type = "Xlib" circle radii are in pixels with minimum one.

  • Colours are interpreted by the viewing application.

For type = "quartz" see the help for quartz.

Note

For type = "Xlib" these devices are based on the X11 device. The colour model used will be that set up by X11.options at the time the first Xlib-based devices was opened (or the first after all such devices have been closed).

Author(s)

Guido Masarotto and Brian Ripley

References

The PNG specification, https://www.w3.org/TR/PNG/.

The TIFF specification, https://www.iso.org/standard/34342.html. See also https://en.wikipedia.org/wiki/TIFF.

See Also

Devices, dev.print

capabilities to see if these devices are supported by this build of R, and if type = "cairo" is supported.

bitmap provides an alternative way to generate plots in many bitmap formats that does not depend on accessing the X11 display but does depend on having GhostScript installed.

Examples

## these examples will work only if the devices are available
## and cairo or an X11 display or a macOS display is available.

## copy current plot to a (large) PNG file
## Not run: dev.print(png, file = "myplot.png", width = 1024, height = 768)

png(file = "myplot.png", bg = "transparent")
plot(1:10)
rect(1, 5, 3, 7, col = "white")
dev.off()

## will make myplot1.jpeg and myplot2.jpeg
jpeg(file = "myplot%d.jpeg")
example(rect)
dev.off()

Copyright (©) 1999–2012 R Foundation for Statistical Computing.
Licensed under the GNU General Public License.